Multielectron-Transfer Kinetics for cis-versus trans-Dioxorhenium(V) Species: Isoelectronic Modeling with Osmium(VI/V) and Control of Interfacial Reactivity by Rhenium(IV) Accessibility

As part of a broader effort aimed at understanding the kinetics of multielectron-transfer processes, especially at electrochemical interfaces, we have been exploring the chemistry of dioxorhenium-(V) species.^1-3 With pyridyl (py) or bipyridyl (bpy) moieties as ancillary ligands, these species are oxidizable by one electron, ^{1a, b, 2h} and reducible by two or even three electrons with uptake of an equivalent number of protons, ^{1a, b, 2h} e.g.:⁴ (formula omitted) In an earlier report on the synthesis, spectroscopy, and electrochemical thermodynamics of the first cis complexes, we noted qualitatively (single-sweep-rate cyclic voltammetry peak separation measurements) that the kinetic reversibility of reaction 1 b is much greater than that of reaction la.^1a We further suggested that the reactivity difference could be due, in part, to differences in thermodynamic accessibility for the kinetic intermediate state, Re(IV).^1a We now wish to report in preliminary detail (1) the mechanisms for electrochemical reduction of cis-and trans-di-oxorhenium(V) species in acidic and neutral solutions and (2) quantitative measures of the electrochemical exchange kinetics at near-neutral pH*.⁵ From (1) and (2) we indeed find that access to Re(IV) is an important factor in differentiating cis/ trans electrochemical kinetics and, further, that a semiquantitative reactivity analysis is possible based on comparisons to isoelectronic osmium species.